Optimal Common-Mode Voltage for Grid-Tied Photovoltaic Cascaded H-Bridge Inverters Under Severe Power Imbalances

This article deals with the control problem of injecting balanced grid currents from a grid-tied photovoltaic cascaded H-bridge (CHB) inverter under severe interphase power imbalances. Existing solutions are hindered by the additional harmonic content required at the inverter output voltages. Therefore, a mathematical formulation for which the solution has minimal harmonic content is proposed. The proposed solution, optimal common-mode voltage (OCMV), has an analytical form that allows deducing and analyzing the CHB operating area. The real-time implementation of the OCMV requires solving a nonlinear two-variables system; thus, an iterative and distributed algorithm is designed. By doing so, the proposed OCMV can be fully formulated and implemented using a real-time control platform. The experimental validation was carried out in a scale-down 3 kW grid-tied seven-level CHB inverter governed by phase-shifted model predictive control. The laboratory results show that the proposed OCMV allows obtaining symmetrical grid currents while maintaining low-distorted inverter output voltages.